Many major projects use digital fieldbus technology as the preferred platform for control and instrumentation. Testing the installation is manual, labour intensive and requires specialist knowledge. With the introduction of online advanced physical layer diagnostic (APLD) equipment, fully automated network testing and reporting reduces the time and cost for commissioning and plant upkeep. This technology optimises the test process and report generation, and helps establish the optimal fieldbus installation quality and thus a highly available automation system. The new APLD equipment needs a revised, yet vastly simplified construction and commissioning procedure, requiring minimal technical expertise.

APLD enables high-speed automated construction as well as commissioning testing with automated test report generation and documentation. APLD provides the “handover” of a system that will have been fully checked to a highly detailed technical level. This is impossible to achieve with methods of manual inspection, thus assuring uncompromised segment quality and system availability for the end customer and plant operator.

Testing and commissioning procedures

Manual inspection

AG-181 is the working guideline created by fieldbus users and issued by the Fieldbus Foundation. The versions up to 3.0 (01/2009) detail manual procedures for installing and commissioning fieldbus segments. Manual inspection requires handling of diverse pieces of equipment such as:

Digital multi-meter for current, voltage and resistance.

Advanced capacitance meter capable of independent RC measurement.

Digital storage oscilloscope.

Handheld fieldbus signal generator and data analyser.

Screwdriver for connect and disconnect of devices and wiring interfaces.

Set of paper test sheets and pens for manual documentation.

The manual inspection prescribes to first run a check on the segment cable and then connect and check each instrument in sequence. Each instrument must be disconnected to repeat this procedure for the next instrument. All documentation for constructional and instrument checks requires manual work. This procedure requires special preparations and has some disadvantages.

These tests demand far more expertise when compared to an equivalent, and require a high level of measurement accuracy. Skilled engineers or technicians manually put together data from disparate devices to interpret the information. While oscilloscope data is extremely useful, to understand many of the potential faults that can occur or exist, it would require intricate specialist knowledge about signal analysis. Additionally, hand-completed paper documents can be prone to errors, omissions or ultimately falsification.

With the arrival and commoditisation of physical layer diagnostic tools, AG-181 was updated in 2012 to include and recommend automatic testing procedures. Planning, installation and wiring guidelines for Profibus PA include respective sections for automated electronic and software support.

With online advanced physical layer diagnostics (APLD), it is now possible to test the entire network automatically. With very little knowledge required, APLD automatically tests many more physical layer attributes and creates software-driven reporting. Simplified result summaries are easy to understand.

A major breakthrough is an automated test engine (ATE). With all devices connected at once, it automatically records many measurements from the physical layer. It can then interpret this data based on a rule-based expert system and provides messages in clear text with potential cause and suggested actions for remedy.

More so, during installation, wiring errors occur infrequently. A testing procedure should account for this by verifying the quality of the installation in very few steps. APLD offer tools for easy fault finding for rapid identification of those faults that do occur.

Stationary APLD is integrated into the power supply as a plug-in module. It continuously monitors the physical layer and issues summary alarms to the operator workstation. Maintenance staff have access to detailed information via Ethernet or fieldbus communications and can thus intervene in a planned and proactive way.

Handheld APLD provides practically the same functionality due to the recent reduction in power consumption and increase in processor power. Measurements in the field are possible without a connection to a laptop, PC or plant asset management system (PAM). These handheld testers store results from measurements and allow for convenient data back-up that is tagged to the actual segment documentation.

The optimised test procedure

The simplified testing procedure essentially tests the network as-built, eliminating the connect and disconnect operations required for individual instrument testing. Instead, all instruments are connected to the segment at once and then automatically tested as shown in Fig. 1. Each segment is tested for:

Compliance or conformance with respective fieldbus guidelines.

Compliance with IEC-61158-2 (fieldbus standard).

Compliance of power supply impedance and compatibility.

Operation, conformance and functionality of cable, devices, terminators, power supplies and protection electronics.

A major advancement is that the installation is tested as-built and then left in place the way it was tested.

Hardware setup of stationary, integrated APLD

Stationary APLD is integrated within the power supply architecture. Wiring or electrically connecting test equipment into each segment is thus eliminated.

Fig. 2 illustrates how the APLD module is integrated within a motherboard-based fieldbus power supply (power hub). Multiple APLD modules are daisy chained via a diagnostic bus, with a capability of monitoring and testing up to 124 segments for each diagnostic bus. The diagnostic bus uses simple RS485 hardware and can be connected to the control room via two ways:

Ethernet interface: This simple technology has the advantage of keeping response time fast on both the primary control system and the fieldbus diagnostic system. A separate path for communications to the APLD enables remote troubleshooting even when a detrimental fault hampers communication on the main line. Setup utility software systems automates the engineering process.

Foundation Fieldbus H1 node integration: Up to 16 APLD modules can communicate to the distributed control system (DCS) albeit at a lower speed and utilizing bandwidth that is needed for process control and instrument diagnostics. Manual engineering is required for this method of integration.

The software on the maintenance station tests and reports on all segments reachable via the diagnostic bus. The interconnecting wiring for the test equipment is minimal. It remains permanently in place, without further connection or disconnection for online diagnostics during the operational lifetime of the plant.

After testing and commissioning, the APLD module continuously monitors the fieldbus physical layer for changes. In such a case, a summary alarm is issued to the operator workstation while the maintenance station provides all details to the case for analysis.

Fig. 2: Connection diagram showing stationary APLD.

Handheld APLD

Where the control system and/or supporting fieldbus power supplies are not on site or cannot be installed, handheld APLD equipment can be utilised. It provides the same level of automation and reporting albeit on a segment by segment test basis. Fig. 3 illustrates that connection to any point of the segment is possible temporarily.

An important use case for handheld APLD is the checkout of partially constructed sites. Instrumentation and wiring work is complete but control room equipment is not yet available. Handheld APLD and a portable, battery-powered fieldbus power supply enable complete segment checkout. The ATE can interrogate devices independently including a comparison of design to as-built information.

This allows for compete field installation and validation before the DCS is available. This is efficient as installation and communications testing can occur while the installation teams are still on site at that time. Corrective action is taken immediately as necessary reducing testing time and effort. Use cases for handheld APLD are:

Checkout of partially constructed sites while installation teams are still available for corrective action.

Detailed analysis for fieldbus experts as a service where APLD are not available.

Quick check at anytime and anywhere.

Verification of the installation quality after modifications or repair work is complete.

Optimsed working procedures for validation of fieldbus installations and automatic documentation of the fully checked system provides assurance for all parties involved.

In the best case, only few and simple wiring errors or installation faults will have been found and corrected with the help of the ATE.

Elimination method

It would be anticipated that only a low percentage of segments display faults. First, primitive failures, such as power supply voltage loss or trunk short circuit need to be ruled out.

The automatic test engine (ATE) creates test reports with cause and action messages. Primitive faults can be easily found and fixed. If the software reports further failures, troubleshooting using a process of elimination has to be performed.

If multiple faults exist in one segment, the information from the test report may on occasion not lead to a satisfactory repair of the fault. This can be the case in situations where the fault is hard to locate such as a ground fault. This method is the quickest method to assess the probable type and position of the fault.

Where the elimination procedure does not conclusively fix the fault, the use of the inline fieldbus oscilloscope can lead to more insight. Detailed oscilloscope data can be viewed for more advanced troubleshooting analysis

(Fig. 4). The fieldbus oscilloscope bridges the gap between automatic diagnostics and manual troubleshooting. A vast selection of trigger point options enables a competent engineer to further assess in-depth information. An oscilloscope with fieldbus-specific trigger options is by far the best tool for the fieldbus expert troubleshooting unusual or complex network faults. An integrated oscilloscope within the diagnostic module has many advantages which are similar to manual inspection procedures. There is no need to read drawings to find the correct terminals, and no cable and junction box disturbance.

Additionally, the digital oscilloscope data can be recorded in a simple way at the maintenance terminal. This way the complete information, test report and oscilloscope traces can be sent to a remote expert for additional troubleshooting advice, saving travel cost and valuable time.

Failure margin checking for commissioning of installed components

The failure margin tests the segment as built for resilience against external disturbances. The ATE can distort the communication through modification of signal amplitude, jitter, and noise until communications fail. This test can be initiated for individual or all connected instruments at once, and is designed to determine in detail the stability of communication per device.

Conclusion

Construction and pre-commissioning/commissioning time saving is a very important consideration for plant start-up. A high level of accuracy, quality and reliability of the work performed is critical to success. Using APLD during construction and pre-commissioning will increase the test performance; reduce time spent on site; accurately record and report findings; no longer require high skillsets; and reduce staffing levels.

Non-intrusive automatic test equipment for construction or commissioning saves significant time, eliminates interference with properly installed cable and performs many more comprehensive test measurements. Automatic test and reporting is accurate and complete. It can accurately and reliably uncover faults that would have been overlooked otherwise, and uncover faults that could cause failure during operation.

Project managers can assess progress effectively. With APLD, they are able to assess repair time as a percentage of testing time and prepare for effortless and cost-effective transition into the operational phase. Finally, the handover to the customer follows a thorough and accurate test sequence at the appropriate time that guarantees quality, performance and reliability.

The stationary APLD system stays in place during operation. History trending and continuous monitoring with alarming integrated in the DCS enables operating and maintenance personnel to respond in a timely and educated manner. Expensive reactive repair work on the fieldbus is eliminated and plant operation is thus improved.